Add explanation of the profile feedback build to the README
Add explanation of the profile feedback build to the README

Also explains that the are additional warnings.

Signed-off-by: Andi Kleen <0474aee45985f5ae829f53849df476200e876990@linux.intel.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git